The present invention relates generally to sizing compounds, and particularly to compounds used to provide water resistance in the paper and textile industries. Specifically, the invention relates to long chain acetal compounds which are useful as sizing agents and novel methods for their production. A particularly preferred long chain, hydrocarbon-containing compound according to the invention is 1,1-octadecoxyoctadecane, the distearyl acetal of stearyl aldehyde. Further, the invention provides two novel and unexpected methods for synthesizing desired acetal compounds.
The paper industry has now largely converted to alkaline papermaking. The two main reasons for this change are the ability to use calcium carbonate (CaCO.sub.3) and the ability to make a stronger and more permanent sheet. A major consequence of this change is that rosin cannot be used as the sizing agent in an alkaine system because rosin must be used with alum and the rosin/alum sizing system does not work at the higher pH range used to make alkaline paper. (An exception to this are dispersed rosin sizes which have been used successfully at operating pH ranges as high as 6.5.) To make alkaline paper, the paper industry has turned to the synthetic reactive sizes, ASA (alkenyl succinic anhydride) and AKD (alkyl ketene dimer). These sizing agents work well in an alkaline environment by reacting with the fiber to form a covalent bond between the sizing compound and the hydroxyl groups on the fiber. The major problem with ASA and AKD is that they also react with water to form an insoluble non-sizing material.
ASA is the largest volume sizing agent now used by the paper industry and is the reaction product of an isomerized alpha olefin (with a chain length of C-16 to C-20) and maleic anhydride. The unsaturation and chain lengths of the starting olefin determine the melting point of the ASA (which is usually less than 9.degree. C.) and it's sizing efficiency. Because ASA is an anhydride, it reacts rapidly with the hydroxyl groups on the fiber to form an ester linkage and with water to form a non-sizing hydrolyzate. The rapid reaction of ASA with fiber makes it the most used sizing agent where sizing is needed before the size press to control pickup, but the rapid reaction with water and the resultant non-sizing hydrolyzate, can cause problems for the paper maker such as press roll picking and machine deposits. ASA is not soluble in water and must be emulsified before use. Most ASA is emulsified using high shear emulsification equipment in the presence of a protective colloid such as starch or a synthetic polymer. Cationic starch such as cationic potato starch is most often used because of cost and the ready availability of wet end starch in most mills.
Because hydrolyzed ASA deposits are a major cause of runnability problems many limitations are imposed on its use. Thus, hydrolysis is minimized by using soft water, cooling the starch to room temperature before making down the emulsion and using the emulsion as soon as possible after it is made. If the emulsion must be stored for any length of time, the temperature must be kept low and the pH should be reduced to between 3 and 4. The ASA also must be added as close to the head box as possible to avoid prolonged contact with water. Finally, good first pass retention is critical so that ASA does not get into the white water system where it will hydrolyze and cause runnability problems. To achieve good first pass retention, retention aids should be used and strict attention must be paid to wet end chemistry. Stock pH and dryer settings are important to maximize size efficiency and minimize deposits.
AKD (alkyl ketene dimer) is the other commonly used cellulose reactive alkaline sizing agent but is less preferred than ASA because of its slow on-machine sizing development and higher cost. AKD is much less reactive than ASA, and can be emulsified by the manufacturer and shipped to the mill as a ready to use product. The lactone ring in AKD reacts with the hydroxyl groups on cellulose to form a .beta.-keto ester on the cellulose fibers. Because the AKD is less reactive than ASA, much of the sizing occurs after the dryer section of the paper machine. In some cases, this may be insufficient to control wet end pick-up. AKD reacts with water to form the .beta.-keto acid which rapidly decarboxylates to form the non-sizing, solid ketone. The reaction of AKD with water occurs very slowly at room temperature, such that emulsified AKD has a shelf life of at least a month. AKD sized sheets can also exhibit other problems such as size reversion, excessive sheet slipperiness and fugitive sizing. Like ASA, AKD should be added as close to the head box as possible.
Thus, while sizing agents currently used in the paper industry work well, they all exhibit some deficiencies. ASA's are very reactive, developing sizing within a few seconds to a few minutes. They are more efficient than rosin, and can be used with alum at pH values as low as 5. However, as ASA's are readily hydrolyzed, emulsions of these products are not very stable and require in-mill emulsification equipment for preparation. This process can be hard to control and the ASA, being very reactive with water as well as cellulose, can hydrolyze with water before it can react with the fiber leaving machine deposits. AKD's are less reactive than ASAs and have more stable emulsions, but are also less reactive with cellulose and may require several hours to develop sizing and usually require the concurrent use of a retention aid. Size reversion, fugitive sizing, and machine deposit problems are also encountered. Both AKD and ASA can make the sheet slippery, resulting in problems for users of reprographic paper. ASA and AKD can also contribute to "telescoping" of the paper reel which can lead to converting problems. Rosin is the lowest cost sizing product, but is much less efficient than ASA or AKD as a sizing agent on a per pound basis. Because of this low efficiency, relatively high dosage rates are required, especially when calcium carbonate is used as the filler. Rosin also does not work well at the higher pH range seen under alkaline papermaking conditions.
Of interest to the present invention is the art relating to the synthesis of acetals and ketals. Ordinarily the general preparative method for the synthesis of an acetal or ketal is the acid catalyzed addition of two moles of alcohol to one mole of aldehyde or ketone, with the formation of one mole of acetal or ketal and one mole of water. These reactions are only acid catalyzed and the products are formed by way of a hemiacetal or hemiketal intermediate. If the aldehyde is small, the reaction proceeds favorably. If the aldehyde is large, water must be removed as the material reacts. The reaction in neither direction is catalyzed by base, so most acetals and ketals are quite stable to bases, though they are easily hydrolyzed by acids (March, Advance Organic Chemistry, 4.sup.th ed; John Wiley & Sons, New York, 1992, p. 889).
Thus, for simple aldehydes, the overall equilibrium constant is favorable, and the acetal may be prepared simply by treating the aldehyde with two equivalents of alcohol and an acid catalyst. With ketones and larger aldehydes, the equilibrium constant for making the acetal or ketal is generally unfavorable. For most aldehydes, the alcohol can be used as the solvent to drive the equilibrium toward completion. For fatty acetals and ketals, the equilibrium constant is generally unfavorable and water must be removed during the reaction to drive the reaction to completion. Another method to prepare acetals and ketals, particularly fatty acetals and ketals is transacetalation of fatty aldehyde dimethyl acetals and fatty alcohols. (See for example, the disclosure of Mahadevan, Lipids, Vol. 5 (No. 6), pp. 563-565 (1969) which discloses a method for the synthesis of a C-14 acetal of fatty aldehydes and fatty alcohols by transacetalation between fatty aldehyde dimethyl acetals and fatty alcohols.) Acetals and ketals are normally stable to basic conditions, but are easily hydrolyzed back to the aldehyde or ketone and the alcohol under acidic conditions.
There is a need in alkaline papermaking for a sizing agent that works in an alkaline environment and does not hydrolyze in the presence of water. This sizing agent should have a low melting point and should develop some sizing before the size press. This new sizing agent should develop a predictable level of sizing and keep this sizing for the life of the sheet. The new product should be safe and made from readily available materials.